12 April 2024 | Julian B. Muñoz, Jordan Mirocha, John Chisholm, Steven R. Furlanetto, and Charlotte Mason
The James Webb Space Telescope (JWST) has revealed that early galaxies are prolific producers of ionizing photons, creating a tension between different methods of measuring reionization. For hydrogen reionization to occur, there must be enough ionizing photons to ionize all hydrogen atoms, including their recombinations. While previous studies suggested that star-forming galaxies could generate these photons, new JWST observations indicate higher ionizing efficiencies and more early galaxies, increasing the number of ionizing photons at high redshifts. Simultaneously, recent low-redshift studies suggest significant escape fractions for faint reionization-era galaxies. Together, these findings suggest that the observed galaxies may end reionization too early, conflicting with cosmic microwave background (CMB) and Lyman-α forest observations. This tension arises because the current galaxy observations imply an excess of ionizing photons, leading to a reionization process that is too fast. Possible solutions include missing astrophysical or observational selection effects, or enhanced recombinations. The paper discusses various avenues to resolve this photon budget crisis, including reevaluating ionizing efficiencies, escape fractions, and the role of recombinations. The findings highlight the need for further observations and theoretical work to reconcile galaxy and CMB observations of reionization.The James Webb Space Telescope (JWST) has revealed that early galaxies are prolific producers of ionizing photons, creating a tension between different methods of measuring reionization. For hydrogen reionization to occur, there must be enough ionizing photons to ionize all hydrogen atoms, including their recombinations. While previous studies suggested that star-forming galaxies could generate these photons, new JWST observations indicate higher ionizing efficiencies and more early galaxies, increasing the number of ionizing photons at high redshifts. Simultaneously, recent low-redshift studies suggest significant escape fractions for faint reionization-era galaxies. Together, these findings suggest that the observed galaxies may end reionization too early, conflicting with cosmic microwave background (CMB) and Lyman-α forest observations. This tension arises because the current galaxy observations imply an excess of ionizing photons, leading to a reionization process that is too fast. Possible solutions include missing astrophysical or observational selection effects, or enhanced recombinations. The paper discusses various avenues to resolve this photon budget crisis, including reevaluating ionizing efficiencies, escape fractions, and the role of recombinations. The findings highlight the need for further observations and theoretical work to reconcile galaxy and CMB observations of reionization.